You look at a herd of cattle and well they all look the same... but they know. They all have an individual personality and those personalities change from day to day. They can have their grumpy days and their happy days and their serene days. But it's unpredictable. You can't be off in outer space when you're dealing with animals.
We don't tend to write about disease in fiction - not just teen novels but all American novels - because it doesn't fit in with our idea of the heroic romantic epic. There is room only for sacrifice heroism war politics and family struggle.